Use a tooltip to explain whitespace errors

Sometime users are confused about why Gerrit shows a particular part
of the new file (which is always green) in red.  Its a whitespace
error, a case that isn't normal.  Explain it with a tooltip message.

Change-Id: Ide90f14c4a25eac5a2bcb1bc9f5e6d67e9ac4cb7
Signed-off-by: Shawn O. Pearce <sop@google.com>

gerrit-prettify/src/main/java/com/google/gerrit/prettify/common/PrettyFormatter.java

@@ -441,14 +441,18 @@ public abstract class PrettyFormatter implements SparseHtmlFile {
 
   private SafeHtml showTabAfterSpace(SafeHtml src) {
     final String m = "( ( |<span[^>]*>|</span>)*\t)";
-    final String r = "<span class=\"wse\">$1</span>";
+    final String r = "<span class=\"wse\"" //
+        + " title=\"" + PrettifyConstants.C.wseTabAfterSpace() + "\"" //
+        + ">$1</span>";
     src = src.replaceFirst("^" + m, r);
     src = src.replaceAll("\n" + m, "\n" + r);
     return src;
   }
 
   private SafeHtml showTrailingWhitespace(SafeHtml src) {
-    final String r = "<span class=\"wse\">$1</span>$2";
+    final String r = "<span class=\"wse\"" //
+        + " title=\"" + PrettifyConstants.C.wseTrailingSpace() + "\"" //
+        + ">$1</span>$2";
     src = src.replaceAll("([ \t][ \t]*)(\r?(</span>)?\n)", r);
     src = src.replaceFirst("([ \t][ \t]*)(\r?(</span>)?\n?)$", r);
     return src;

gerrit-prettify/src/main/resources/com/google/gerrit/prettify/client/gerrit.css

@@ -21,6 +21,7 @@
 
 .wse {
   background: red;
+  cursor: pointer;
 }
 
 .vt {